Sample transfer devices of this kind are utilized in particular in electron microscopy. Samples to be investigated, which contain e.g. cells, enzymes, viruses, or lipid layers, are cryofixed, i.e. the water-containing sample is frozen very quickly to temperatures below −150° C., avoiding the formation of ice crystals. The biological structures can thereby be kept in their natural state. For example, a biological process can be halted at any desired point in time by cryofixation and can be investigated, for example, in a cryo-electron microscope and/or in a light microscope with corresponding sample cooling. Prior to the actual investigation, cryofixed samples can be subjected to further preparation steps in a manner known per se, for example processing using freeze-fracturing, freeze-etching, and/or coating techniques.
In order not to impair the quality of the frozen samples, it is very important that they be transferred in contamination-free, and optionally cooled, fashion between the processing devices or analytical devices that are being used. “Processing devices” or “processing units” may be understood, for example, as a cryofixation device, a freeze-fracture apparatus, a freeze-etching apparatus, or a coating apparatus, while “analytical devices” or “analytical units” are to be understood, for example, as a cryo-electron microscope or a cooled light microscope.
So-called sample transfer devices are used for the purpose of conveying or transferring a sample to a processing unit or analytical unit. One such sample transfer device is represented, for example, by the “Leica EM VCT100” vacuum cryotransfer system (manufacturer: Leica Microsystems), which is described in the brochure of the same name that is accessible via the link: http://www.leica-microsystems.com/fileadmin/downloads/Leica%20EM%20VCT100/Brochures/Leica_EM VCT100_Brochure_EN.pdf. This transfer system comprises a transfer rod for detachable mounting of a sample holder, the latter being arranged at the end of the transfer rod. If a very small frozen electron microscopy sample is present on the sample holder, the latter can be picked up by suitable displacement of the transfer rod and by connection to the sample holder, the sample then being arranged, by another (backward) displacement of the transfer rod, in a chamber of the transfer system for the purpose of transferring the sample to a processing unit or analytical unit. In the chamber, the sample is kept under inert gas or under high vacuum and, to the extent necessary, at the low temperature necessary for further cryo-processing or cryo-investigation. The transfer system attachment system is configured in such a way that a high-vacuum connection to the processing unit or analytical unit can thereby be created. The chamber or the sample holder is furthermore in communication with a coolant reservoir, usually a Dewar container, that can be filled with a coolant, typically liquid nitrogen. The sample holder, and the sample present thereon, are thereby cooled.
A plurality of different sample holders are available depending on the particular processing unit or analytical unit, and are depicted in the aforesaid brochure. After cryofixation of the sample, the latter is loaded into the vacuum transfer system with a suitable sample holder. Transfer to the downstream processing unit or analysis unit then occurs. The transfer is accomplished in a cooled state, so that the sample cannot incipiently or completely thaw and thus become unusable. Contamination, for example upon exposure of the sample to ambient air, is also to be avoided. Sample transfer devices, such as the above-described “Leica EM VCT100” vacuum cryotransfer system, comprise vacuum sliders or slide valves so that the sample can be introduced, for example under vacuum, into the corresponding processing device or analytical device. For example, a slide valve is arranged at the attachment point of the sample transfer device, and a further slide valve at the corresponding attachment point of the processing unit or analytical unit. After attachment of the sample transfer device to the processing unit or analytical unit with the respective slide valves closed, the cavity that is produced is evacuated in the manner of an air lock. The slide valves are then opened, and the sample is then transferred under vacuum to the processing unit or analytical unit. For this, the transfer rod can be displaced linearly and often also rotated around its axis.
It has been found that changes in the state of the sample during transfer can lead to damage to the sample, and thus misinterpretations or unusable results upon analysis, or can complicate subsequent processing steps or make them entirely impossible. Sample transfer is often accomplished in time-controlled fashion based on empirical values.